(1) Field of the invention
The present invention relates to a method and apparatus for detecting magnetic disk defects in which the defects can be rapidly detected.
When data is read out from a magnetic disk having defects, such as minute defects in the magnetic film formed on a base material of the magnetic disk, the amplitude of the read signal rapidly decreases, as shown by symbol A in FIG. 1A. In addition, when there are defects in an area on the magnetic disk from which data is erased, the data is not quite erased from the area and the amplitude of a background signal read out from the area increase sharply to a level greater than the noise level, as shown by symbol B in FIG. 1B. Thus, the magnetic disk defects can be detected based on the above variation of the amplitude of the read signal.
(2) Description of the related art
Conventionally, the defects are detected by a tester as shown in FIG. 2.
Referring to FIG. 2, a magnetic disk 1 to be tested is mounted on a spindle shaft 3 rotated by a spindle motor 2. A magnetic head 4 is mounted on an end of an actuator 7 moved by a stepping motor 16. The magnetic head 4 is connected to a head IC 8 so that a read signal output from the magnetic head 4 is supplied to the head IC 8 and a write signal supplied from the head IC 8 is written on the magnetic disk 1 via the magnetic head 4. The head IC 8 is driven by a driver circuit 9. The read signal received by the head IC8 is transmitted to a defect detecting circuit 11 via the driver circuit 9 and a main amplifier 10. Test data output from the defect detecting circuit 11 is supplied to the head IC 8 via the main amplifier 10 and the driver circuit 9. The test data is supplied, as the write data, from the head IC 8 to the magnetic head 4. A CPU (Central Processing Unit) 12 controls the defect detecting circuit 11. That is, the CPU 12 supplies various instructions and control information to the defect detecting circuit 11 and receives detecting result data generated by the defect detecting circuit 11. The CPU 12 is connected to a printer 13 so that the detecting result data supplied from the defect detecting circuit 11 is printed out by the printer 13.
The defect detecting circuit 11 is formed of a digital signal processor and detects defects of the magnetic disk 1 based on the read signal obtained by the reading of the test data. The defect detecting circuit 11 performs the following processes.
1 First process
Test data having a predetermined frequency (e.g. the maximum frequency 2F) is written on the i-th track (i=0,1, . . . ) of the magnetic disk 1.
2 Second process
The test data is read out from the i-th track of the magnetic disk 1. Amplitude levels (peak levels) of a read signal obtained from the magnetic head 4 when the test data is read out are averaged for the i-th track, and an average level Av(i) of the read signal for the i-th track is obtained. In addition, it is checked whether or not a missing-pulse is generated. The missing-pulse is generated when the amplitude of the read signal is less than a predetermined threshold level TH.sub.H. That is, when the amplitude of the read signal rapidly decreases as shown in FIG. 1A, the missing-pulse is to be generated. The threshold level TH.sub.H is decided based on the average level Av(i-1) which has already been obtained in the (i-1)-th track. The threshold level TH.sub.H is, for example, 65% of the average amplitude level Av(i-1). By this process, a defect in the i-th track can be detected.
3 Third process
The test data is erased from the i-th track of the magnetic disk 1.
4 Fourth process
A signal (a background signal) is read out from the i-th track from which the test data has been erased. It is checked whether or not an extra-pulse is to be generated. The extra-pulse is generated when the amplitude of the background signal is greater than a predetermined threshold level TH.sub.L. That is, when the amplitude of the background signal rapidly exceeds the noise level, as shown in FIG. 1B, the extra-pulse is generated. The threshold level TH.sub.L is decided based on the average level Av(i) which has already been obtained for the i-th track. The threshold level TH.sub.L is, for example, 25% of the average amplitude level Av(i). By this process, a defect in the i-th track can be detected.
5 Fifth process
A seek operation is performed so that the magnetic head 4 moves from the i-th track to the (i+l)-th track.
After this, the processes 1 through 5 (one cycle) are repeated until the processes for the last track of the magnetic disk 1 are completed.
Each of the above processes requires one revolution of the magnetic disk 1. Thus, to detect a defect in one track, the magnetic disk 1 must be rotated by at least five revolutions, as shown in FIG. 3. In FIG. 3, a symbol I indicates an index of a track and the axis of abscissas indicates the number of revolutions of the magnetic disk 1. In addition, the erasing of the test data must be performed, for every track, to check the extra-pulse.